In a conventional control system, generally, a PLC is placed at the center of control, and a plurality of PLCs and host computers are connected via predetermined interface circuits directly or indirectly via dedicated adapters, so that transmission of control data between the PLCs as well as between the PLCs and the host computers is carried out with use of communication functions of the PLCs.
However, developed from a sequencer utilizing a relay, a PLC best performs in operations related with input/output control, such as an ON/OFF control of a switch and an operation of taking in data from a sensor, and use of a PLC as a center of data communication will overload the PLC, even though such use is possible.
Furthermore, data communication time becomes excessively long, for instance, upon an initial setting before control when more data transmission is required, as compared with during control. In a case where a personal computer is directly connected to a display device of a PLC to shorten the data communication time, connection change becomes extremely bothersome.
The following description will explain a typical conventional control device, while referring to FIG. 24 that is a block diagram schematically illustrating an arrangement of the same. A plurality of PLCs (three PLCs in the case of FIG. 24), that is, PLCs 501α, 501β, and 501γ are equipped with their dedicated display devices 502α, 502β, and 502γ, respectively, and are connected with a host computer 504 via a network circuit 503. In response to a control output of the host computer 504, the PLCs 501α, 501β, and 501γ control one or a plurality of apparatuses to be controlled (not shown), receive states of the control-target apparatuses from sensors not shown, and cause the display devices 502α, 502β, and 502γ to display the states.
The foregoing PLC 501α, 501β, and 501γ may use different protocols, as indicated with the reference codes of α, β, and γ in FIG. 24, depending on respective manufacturers thereof, or in some cases, from one PLC to another PLC even though made of the same manufacturer. Therefore, the host computer 504 conducts communication with the PLCs 501α, 501β, and 501γ via the network circuit 503 according to protocols α, β, and γ, respectively.
Therefore, the display devices 502α, 502β, and 502γ are connected with the corresponding PLCs 501α, 501β, and 501γ via connection cables 505α, 505β, and 505γ, respectively. The display devices 502α, 502β, and 502γ synthesize data expressing control states transmitted according to the foregoing protocols α, β, and γ, respectively, with characters and image data installed beforehand, so as to produce display screens and execute display. More specifically, for example, changeable data such as data expressing the foregoing control states are synthesized with a fixed screen display in which names of the foregoing apparatuses to be controlled are added to a background illustrating a manufacturing line, and a resultant screen display is displayed.
Therefore, the foregoing display screen is changed upon a change of products being produced or a producing method, and also it is changed out upon occurrence of an inconvenience.
As described above, the foregoing PLCs 501α, 501β, and 501γ have originally been developed from control-use sequencers using relays, and they excel in ON/OFF control of switches and functions as input/output circuits for taking in data from sensors, whereas they are inferior in data communication functions. Therefore, they are capable of transmitting data like data expressing control states of the apparatuses to be controlled to the display devices 502α, 502β, and 502γ, but they are not suitable for transmission of enormous data such as character and image data.
Therefore, conventionally, upon changes of the display screen, a personal computer 506 storing the character and image data is taken into the manufacturing site, and the display devices 502α, 502β, and 502γ set in walls or manufacturing machines are taken out. After the PLCs 501α, 501β, and 501γ are turned into a offline state, disconnected to the network circuit 503, the connection cables 505α, 505β, and 505γ connecting the display devices 502α, 502β, and 502γ with the PLCs 501α, 501β, and 501γ are disconnected from the display, devices 502α, 502β, and 502γ (indicated by an arrow code φ as to the connection cable 505α). Thereafter, a connection cable 507 extended from the personal computer 506 is connected thereto (indicated by an arrow code A as to the connection cable 507), so that the foregoing character and image data are installed.
Upon an end of the installation of data, connection of the connection cables 505α, 505β, and 505γ with the display devices 502α, 502β, and 502γ is restored, the display devices 502α, 502β, and 502γ are returned to their own places in the walls or manufacturing machines, and the PLCs 501α, 501β, and 501γ are returned into an online state, with connection of the same with the network circuit 503 restored.
Therefore, the operation of changing the screen display is complicated, and it is necessary to consecutively connect the personal computer 506 with the display device whose screen display is to be changed, one after another, for downloading. Therefore, there arises a problem that an offline time is prolonged.
Furthermore, transmission of a relatively large amount of data upon, for example, a change of processed products or drastic changes to processing operation data is also executed by connecting the personal computer 506 with the PLCs 501α, 501β, and 501γ and installing preset data such as application programs and initial values, like in the foregoing case of the foregoing characters and image data.
Furthermore, since the PLCs differ in communication protocols for data communication, depending on respective manufacturers thereof, or in some cases from one PLC to another PLC even though produced by the same manufacturer, there are innumerable communication protocols to which a host computer has to be suited, and it is necessary to anew prepare communication protocols that are suited for the PLCs, for dedicated use of the host computer. Thus, there exists a problem of extreme inconvenience.